Powder coating compositions are well known in the industry and have been prepared by various methods. The use of powder coatings has grown dramatically primarily due to their environmental advantages over liquid coatings, e.g. solvent based coatings. Specifically, powder coatings do not contain volatile organic solvents that evaporate during application or curing; omitting solvent results in considerable environmental and costs savings. For example, conditioned air from powder paint booths may be recycled rather than exhausted because it does not contain solvent vapor. Further, powder coating overspray is easily captured and recycled without the use of a water-wash system, eliminating environmentally difficult paint sludge from booth wash water.
Thermosetting coating powders are typically made by first blending or “dry-mixing” the starting materials in a batch mixer, also called a pre-hopper. This “premix” is then melt compounded or melt mixed in an extruder, such as a single- or twin-screw extruder. In the extruder, the starting materials melt and are further blended together to form a homogeneous mixture, also called a “melt mix”. A typical extruder will have heat applied to the extruder along the entire length thereof (except perhaps at the intake spot) to maintain an elevated temperature of the materials during melt mixing. The temperature is selected above the melt temperature of the resin but below the temperature that would cause significant crosslinking to occur. The temperature is determined on a product-by-product basis and will depend upon the specific constituents of a given powder coating as well as the specifics of the extruder (e.g. screw pitch, bore diameter, etc.). It is desired that minimal reaction occur between the resin and curing agent(s) in the extruder. As the melt mix exits the extruder as “extrudate”, it is cooled rapidly on a cooled drum and then passed to a cooled belt. The cooled compound is broken into granules. The friable granules are then ground in a hammer mill, or the like, to a fine particle size that may be further processed, such as by being screened in a classifier, before packaging.
The dry mixing and melt mixing of a typical powder coating manufacturing process may be insufficient to disperse the pigment uniformly throughout the powder coating or may otherwise result in a less homogeneous powder coating than desired. The insufficient mixing may result in non-uniform flow of the powder during application of the coating, and may further result in poor gloss and distinctness-of-image in the final coating. Increasing the time in the extruder to provide more mixing could improve homogeneity, but also may increase the amount or degree of internal crosslinking of the extrudate. Therefore, there remains a need for methods of producing pigmented and/or non-pigmented powder coating compositions that disperse pigment and/or other dry constituents uniformly throughout the composition without detrimentally affecting the extrudate.